CN210243962U

CN210243962U - Optical structure for augmented reality display and augmented reality display

Info

Publication number: CN210243962U
Application number: CN201920510344.5U
Authority: CN
Inventors: Salim Valera Mohmed; 穆罕默德·萨利姆·瓦莱拉
Original assignee: Wave Optics Ltd
Current assignee: Snap Inc
Priority date: 2018-05-17
Filing date: 2019-04-15
Publication date: 2020-04-03
Anticipated expiration: 2029-04-15
Also published as: EP3794400B1; US20230341696A1; GB201808055D0; WO2019220072A1; TWI791822B; US20210239982A1; TW201947263A; EP3794400A1; US11782275B2; GB2573793A

Abstract

An optical structure for an augmented reality display and an augmented reality display are disclosed. The color projector (2) emits an image in a narrow light beam including three primary colors, i.e., red, green, and blue. A pair of waveguides (4, 6) is provided in the path of the projected light beam. The first input grating (8) receives light from the projector (2) and diffracts the received light such that the first and second primary color wavelengths of the diffracted light are coupled into the first waveguide (6) and such that the second and third primary color wavelengths of the diffracted light are coupled out of the first waveguide in a direction towards the second waveguide 4. A second input diffraction grating (10) receives light coupled out of the first waveguide (6) and diffracts the second and third primary colors so that they are coupled into the second waveguide (4).

Description

Optical structure for augmented reality display and augmented reality display

Technical Field

The present invention relates to optical structures for use in full-color augmented reality or virtual reality displays (e.g., headphones or head-up displays). In particular, the present invention relates to a pair of waveguides having diffractive optical elements that separate the three primary colors from the projector between the two waveguides.

Background

Augmented reality displays enable users to see their surroundings as well as projected images. In military or transportation applications, the projected image may be superimposed on the real world as perceived by the user. Other applications of these displays include video games and wearable devices (e.g., glasses). In contrast, in virtual reality displays, users can only perceive projected images while light from their real world environment is occluded.

In a typical augmented reality device, a transparent display screen is provided in front of the user so that they can still see the physical world. The display screen is typically a glass waveguide and is provided with a projector on one side. Light from the projector is coupled into the waveguide by a diffraction grating (input grating). The projected light is totally internally reflected within the waveguide. Another diffraction grating (output grating) then couples the light out of the waveguide so that it can be seen by a user. The projector may provide information and/or images that enhance the user's perspective of the physical world.

One challenge in the field of augmented reality displays is providing full color images. To date, it has proven difficult to avoid introducing undesirable optical effects. In one approach, colored light is separated into three basic components: red, green and blue. Each fundamental component is then separately coupled towards the viewer via a dedicated waveguide. This requires a stack of three waveguides, which for some applications may become undesirably bulky.

WO 2011/131978 describes an optical element which attempts to provide full colour images using only two waveguides. According to this technique, to provide full color viewing, the three primary color components are separated between two waveguides and recombined into light that is provided toward the viewer. This technique requires the use of diffraction gratings with small periods, which are difficult to manufacture.

It is an object of the present invention to provide an alternative technique for providing full color images using only two waveguides, which can be achieved using simpler manufacturing principles.

SUMMERY OF THE UTILITY MODEL

According to an aspect of the present invention, there is provided an optical structure for use in an augmented reality display, the optical structure comprising a first waveguide and a second waveguide, wherein the first waveguide comprises: a first input diffractive optical element configured to receive light from the projector and diffract the received light such that first and second primary color wavelengths of the diffracted light are coupled into the first waveguide to be totally internally reflected within the first waveguide, and such that second and third primary color wavelengths of the diffracted light are coupled out of the first waveguide in a direction toward the second waveguide; and a first output diffractive optical element configured to receive and diffract the totally internally reflected light within the first waveguide to couple the totally internally reflected light out of the first waveguide towards a viewer; wherein the second waveguide comprises: a second input diffractive optical element configured to receive diffracted light coupled out of the first waveguide by the first input diffractive optical element and diffract the received light such that second and third primary color wavelengths of the diffracted light are coupled into the second waveguide to be totally internally reflected within the second waveguide; and a second output diffractive optical element configured to receive and diffract the totally internally reflected light within the second waveguide to couple the totally internally reflected light out of the second waveguide towards a viewer.

In this arrangement, the first and second input diffractive optical elements are laterally offset from each other relative to an input direction of light coupled by the projector toward the first waveguide. In this way, light can be projected onto the first input diffractive optical element without first interacting with the second input diffractive optical structure.

The present invention provides techniques for coupling three primary colors into two stacked waveguides. It is unusual for the present technology to couple light that has been diffracted by an input diffractive optical element on a first waveguide into a second waveguide. This may advantageously reduce the manufacturing complexity of the second input diffractive optical element.

Preferably, the first input diffractive optical element is a reflective diffraction grating. Alternatively, the first input diffractive optical element may be provided as a transmissive diffraction grating. However, the reflective arrangement is preferred as it can be arranged with greater efficiency. The second input diffractive optical element may be a transmissive diffraction grating.

Preferably, the diffractive optical element has a period defined by the spacing of the diffractive optical features. The reciprocal of the period of the second output diffractive optical element is preferably equal to the reciprocal of the period of the first input diffractive optical element plus the reciprocal of the period of the second input diffractive optical element. In other words, the grating vectors of the first and second input diffractive optical elements are added to obtain the grating vector of the second output diffractive optical element. Preferably, the grating vector of the first output diffractive optical element is equal to the grating vector of the first input diffractive optical element. In this way, light output from the first and second output diffractive optical elements toward the viewer is conjugate to light input from the projector to the optical structure. This ensures that full color light can be provided without introducing undesirable optical effects.

In one embodiment, the period of the first input grating may be about 440nm and the period of the second output grating may be about 335 nm. This means that the period of the second input grating should be about 1400nm, which is clearly easier to manufacture than a grating with a smaller period.

Preferably, the first primary color, the second primary color and the third primary color are red, green and blue, respectively. In other embodiments, different primary colors may be used, for example, yellow, cyan, and magenta.

According to the utility model discloses an on the other hand provides an augmented reality display, include: an optical structure as hereinbefore defined; and a projector configured to direct light having a first primary color, a second primary color, and a third primary color toward the first input diffractive optical element.

Drawings

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which:

fig. 1 is a side view of an augmented reality display in an embodiment of the invention;

fig. 2 is a side view of an augmented reality display in another embodiment of the present invention; and

fig. 3 is a side view of an augmented reality display in another embodiment of the invention.

Detailed Description

Fig. 1 is a side view of an augmented reality display. A color projector 2 is provided which emits an image in a narrow beam including three primary colors (i.e., red, green, and blue). A pair of

waveguides

4, 6 is arranged in the path of the projected light beam. The

waveguides

4, 6 are optically transparent and are made of glass with a refractive index n of about 1.7 in this embodiment. As shown in fig. 1, the

waveguides

4, 6 have a first and a second main surface parallel to each other and extending out of the plane of the page. For simplicity, the waveguides are referred to as the "blue" waveguide 4 and the "red" waveguide 6. This does not refer to the color of the waveguides themselves, as they are optically transparent. But rather refers to the dominant color of light that the respective waveguides are configured to carry.

Light rays along the optical axis of the projector 2 are transmitted directly through the lucent blue waveguide 4 and through the first surface of the red waveguide 6 at an angle parallel to the surface normal of the blue waveguide 4. From the perspective of the projector 2, a reflective input diffraction grating 8 with a period of 440nm, defined as the distance between the individual grating grooves, is placed on the rear surface of the red waveguide 6. The reflective input diffraction grating 8 is configured to diffract light incident thereon. The diffraction angle is of course wavelength dependent according to the grating equation. Thus, the angular spread emitted away from the reflective input diffraction grating 8 provides a range of colors.

The period of the reflective input diffraction grating 8 is chosen such that after diffraction the red wavelength is totally internally reflected within the red waveguide 6. A portion of the green wavelengths are also totally internally reflected within the red waveguide 6. However, blue wavelengths are diffracted at angles less than the critical angle for total internal reflection. Thus, the blue and some of the green wavelengths of light escape total internal reflection within the red waveguide 6 to be refracted. The wavelengths of the refracted light are propagated into the air gap between the red 6 and blue 4 waveguides and received at the second input diffraction grating 10 on the surface of the blue waveguide 4.

The second input diffraction grating 10 is provided with a period of about 1404 nm. The second input diffraction grating 10 is configured to diffract the received light such that the received blue and green wavelengths are coupled into the blue waveguide 4 at an angle greater than the critical angle for total internal reflection. Thus, the blue and green wavelengths are diffracted twice: first diffracted by the first input diffraction grating 8 and then by the second input diffraction grating 10 such that these wavelengths are totally internally reflected within the blue waveguide 4. The light received at the second input diffraction grating 10 has been angularly tilted and this allows the 1404nm periodic grating to be optimised using a square mode structure rather than a tilted structure. In addition, since the period of the second input diffraction grating 10 is a multiple of the blue and green wavelengths, techniques such as "area-coded" gratings may be used, as described, for example, in Optics Communications 266 (2006), 697-703.

The light that is totally internally reflected within the red waveguide 6 extends towards the first output diffractive optical element 14. The first output diffractive optical element 14 is arranged to have a relatively low efficiency so as to act as an expansion grating and provide expansion of the beam in at least one dimension (and preferably in two dimensions). The second output diffractive optical element 12 is similar to the first output diffractive optical element 14, but is provided with a different period. In this example, the first output diffractive optical element 14 has a period of about 440nm and the second output diffractive optical element 12 has a period of about 335 nm.

The first input diffraction grating 8 and the second input diffraction grating 10 are staggered from each other with respect to the direction of light output by the projector 2. In an alternative embodiment, as shown in fig. 2, the blue waveguide 4 is disposed at a lower position so that light from the projector 2 is received directly at the red waveguide 6 without first passing through the blue waveguide 4.

The first output diffractive optical element 14 and the second output diffractive optical element 12 in this embodiment are extended diffractive optical exit pupils as described in WO 2016/020643. Thus, the first output element 14 and the second output element 12 comprise a pair of crossed linear gratings or photonic crystal structures. In both cases, the

output elements

12, 14 each comprise two diffractive optical elements which overlap one another in the waveguide 2 or on the waveguide 2. Each diffractive optical element within the

respective output element

12, 14 may then couple the received light towards the other diffractive optical element of the pair, which may then act as a diffractive optical element coupling the light out of the red waveguide 4 or the blue waveguide 6 towards the viewer. The grating period for the superimposed diffractive optical element of the first output diffractive optical element 14 in the red waveguide 6 is 440nm, while the grating period for the superimposed diffractive optical element of the second output diffractive optical element 12 in the blue waveguide 4 is 335 nm.

In an alternative configuration, the first output diffractive optical element 14 and the second output diffractive optical element 12 may be simple linear diffraction gratings to provide one-dimensional expansion of the totally internally reflected light within the red 4 and blue 6 waveguides.

It is important that undesirable optical effects are avoided in the present arrangement and that the colour image emitted by the projector 2 is provided in output to the viewer. This is achieved, in part, by careful selection of the grating periods of the first and second

input diffraction gratings

8, 10 and the first and second output diffractive

optical elements

14, 12. In particular, it has been found important to maintain the beam conjugate. This is achieved in the red waveguide 6 because the period of the first input diffraction grating 8 is the same as the period of the first output diffractive optical element 14. An additional consideration is required for the blue waveguide 4 because the light has been diffracted once when it is received at the second input diffraction grating 10. A grating vector may be defined for each grating with a magnitude equal to the inverse of the grating period. For the blue waveguide 4, the magnitude of the grating vector of the first input diffraction grating 8 plus the magnitude of the grating vector of the second input diffraction grating 10 should be equal to the magnitude of the grating vector of the second output diffractive optical element 12.

Fig. 3 is a side view of an augmented reality display in another embodiment of the invention. In this embodiment, the positions of the blue waveguide 4 and the red waveguide 6 are interchanged compared to the embodiment of fig. 1 and 2. Light from the projector 2 is initially incident on the red waveguide 6 and, in this embodiment, the first input diffraction grating 8 is provided as a transmission grating. The first input diffraction grating 8 diffracts the light such that the red and some of the green wavelengths of the light couple into the red waveguide 6 to be totally internally reflected within the red waveguide 6. The blue and some of the green wavelengths of light are diffracted by the first input diffraction grating 8 at an angle smaller than the critical angle for total internal reflection. These wavelengths therefore escape total internal reflection and are refracted into the air gap towards the blue waveguide 4. In this exemplary embodiment, the second input diffraction grating 10 on the blue waveguide 4 is provided as a transmission grating on the front surface of the blue waveguide 4 from the perspective of the projector 2. The second input diffraction grating 10 is configured to diffract the blue and green wavelengths at an angle greater than the critical angle for total internal reflection within the blue waveguide 4. Thus, blue (and some green) wavelengths are coupled into the blue waveguide 4 to be totally internally reflected within the blue waveguide 4.

The red waveguide 6 comprises a first output diffractive optical structure 14 and the blue waveguide 4 comprises a second output diffractive optical structure 12, and these optical structures have similar characteristics to those described in relation to the previous embodiments.

Claims

1. An optical structure for an augmented reality display, the optical structure comprising a first waveguide and a second waveguide,

wherein the first waveguide comprises:

a first input diffractive optical element configured to: receiving light from a projector and diffracting the received light such that first and second primary color wavelengths of the diffracted light are coupled into the first waveguide to be totally internally reflected within the first waveguide, and such that second and third primary color wavelengths of the diffracted light are coupled out of the first waveguide in a direction towards the second waveguide; and

a first output diffractive optical element configured to receive and diffract totally internally reflected light within the first waveguide to couple the totally internally reflected light out of the first waveguide towards a viewer;

wherein the second waveguide comprises:

a second input diffractive optical element configured to receive diffracted light coupled out of the first waveguide by the first input diffractive optical element and diffract the received light such that second and third primary color wavelengths of the diffracted light are coupled into the second waveguide to be totally internally reflected within the second waveguide; and

a second output diffractive optical element configured to receive and diffract the totally internally reflected light within the second waveguide to couple the totally internally reflected light out of the second waveguide toward the viewer.

2. The optical structure of claim 1, wherein the first input diffractive optical element is a reflective diffraction grating.

3. An optical structure according to claim 1 or 2, wherein the second input diffractive optical element is a transmissive diffraction grating.

4. An optical structure according to any preceding claim wherein the diffractive optical element has a period that is a spacing of the diffractive optical features and wherein the inverse of the period of the second output diffractive optical element is equal to the inverse of the period of the first input diffractive optical element plus the inverse of the period of the second input diffractive optical element.

5. The optical structure according to any one of the preceding claims, wherein the first primary color, the second primary color and the third primary color are red, green and blue, respectively.

6. An augmented reality display comprising:

an optical structure according to any of the preceding claims; and

a projector configured to direct light having a first primary color, a second primary color, and a third primary color toward the first input diffractive optical element.